Quantum cryptography has been proposed as a solution to the cryptographic key distribution problem with absolute security guaranteed by the principles of quantum mechanics. Through this scheme it is possible to find out whether a spy tried to eavesdrop on the transmission, which was impossible to discover using classical transmission techniques. In this work a brief review of quantum cryptography theory, transmission techniques and technological problems involved were performed. It was analyzed in detail the transmission technique employing frequency coding, and a comparison was made between the different modulation schemes and the BB84 and B92 protocols. It was demonstrated that the two existing modulation formats (AM-AM and PM-PM) are in fact equivalent and a new format (AM-PM) was proposed, the only one able to accommodate classical BB84. Classical measurements were performed on the AM-AM and AMPM formats.